Inhalers are used to administer pre-determined quantities (doses) of inhalable dry powder medicament to the lungs of a patient. Generally, inhalers are mechanical systems that generate a metered cloud of medicament that is inhaled by a patient. Many of these prior art inhaler devices use chloroflourocarbon (CFC) gas to facilitate generation of the metered cloud of medicament. However, since CFCs are no longer used in consumer products, other techniques for generating the medicament cloud have been explored.
One example of a non-CFC, prior art inhaler is disclosed in U.S. Pat. No. 4,811,731 issued Mar. 14, 1989 (the "'731 patent"). This patent discloses an inhaler that contains a plurality of measured doses of medicament stored in a blisterpack. Upon use, one of the blisters in the blisterpack is punctured and a patient inhales the medicament from the punctured blister via a mouthpiece of the inhaler. In the '731 patent, the medicament dosage varies with the amount of force with which the patient inhales. Since inhalation of a powder from a blisterpack is rather difficult and a patient does not repeatedly inhale with the same force each time the medication is taken, the medicament dosage that is actually consumed can vary greatly from dose to dose.
Therefore, a need exists in the art for an inhaler that, over a wide range of inspirable flow rates, maximizes drug propagation to the lungs and provides, with each use of the inhaler, substantially identical doses of medicament to the lungs.